[boost] Patch for Identification of NVCC with Changing Backends

Dear Members, I often program CUDA applications and use Boost with comforts. And I found Boost did not work fine when NVCC used a user-defined backend C++ compiler with the -ccbin option. The current implementation of Boost only identifies GCC and Visual C++ that are the default backend compilers of NVCC. So I wrote a patch for other backend compilers. It ports the generic identification routine of select_compiler_config.hpp to nvcc.hpp. Best regards, Ryohei